Common Units and Conversions

Energy use in the United States involves many diverse industries and sectors, each of which uses its own conventions and units to describe energy production and use. Although these units are in common usage throughout the energy industry, they are not always consistent and are not well understood by nonexperts. Similarly, different types of units are employed to describe emissions resulting from energy-related use activities. This appendix describes the units used for principal energy supply and consumption activities and provides some useful conversion factors. The U.S. Department of Energy’s Energy Information Administration website provides additional information about energy (see www.eia.doe.gov/basics/conversion_basics.html) units and conversion factors, including easy-to-use energy conversion calculators. Total U.S. energy use in 2007 was 101.5 quadrillion (1015) Btu or 96 Exa (1018) Joules.

Electricity

Electrical generating capacity is power and expressed in units of kilowatts (kW), megawatts (MW = 103 kW), and gigawatts (GW = 106 kW). It is defined as the maximum electrical output that can be supplied by a generating facility operating at ambient conditions. Coal power plants typically have generation capacities of about 500 MW; nuclear plants about 1,000 MW (1 GW); intermittent sources (e.g., natural gas peaking plants and individual wind turbines) about one to a few megawatts; and residential roof-top installations of solar photovoltaics about a few kilowatts.

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Common Units and Conversions ."
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Common Units and Conversions
Energy use in the United States involves many diverse industries and
sectors, each of which uses its own conventions and units to describe energy
production and use. Although these units are in common usage throughout
the energy industry, they are not always consistent and are not well un-
derstood by nonexperts. Similarly, different types of units are employed to
describe emissions resulting from energy-related use activities. This appen-
dix describes the units used for principal energy supply and consumption
activities and provides some useful conversion factors. The U.S. Department
of Energy’s Energy Information Administration website provides additional
information about energy (see www.eia.doe.gov/basics/conversion_basics.
html) units and conversion factors, including easy-to-use energy conversion
calculators. Total U.S. energy use in 2007 was 101.5 quadrillion (1015) Btu
or 96 Exa (1018) Joules.
Electricity
• Electrical generating capacity is power and expressed in units of
kilowatts (kW), megawatts (MW = 103 kW), and gigawatts (GW = 106
kW). It is defined as the maximum electrical output that can be supplied
by a generating facility operating at ambient conditions. Coal power plants
typically have generation capacities of about 500 MW; nuclear plants about
1,000 MW (1 GW); intermittent sources (e.g., natural gas peaking plants
and individual wind turbines) about one to a few megawatts; and residen-
tial roof-top installations of solar photovoltaics about a few kilowatts.
• Electricity supply and consumption is expressed in units of kilowatt
40

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406 HIDDEN COSTS OF ENERGY
hours (kWh), megawatt hours (MWh), gigawatt hours (GWh), or terawatt
hours (TWh) (109 kWh). One kWh is equal to a power of 1,000 watts (the
typical electricity that is consumed by a hand-held hair dryer) supplied or
consumed over the period of an hour. Annual total delivered electricity in
the United States is about 4,000 TWh and the average annual electricity
consumption per U.S. household is about 11,000 kWh.
o 1 kWh of electricity is equivalent to 3,410 Btu of thermal energy
if the conversion has no inefficiencies.
o In a 33% efficient power plant, 10,230 Btu of input primary
energy are required to produce 1 kWh of electricity.
Fossil Fuels and Other Liquid Fuels
• Coal supply and consumption in the United States is usually ex-
pressed in units of metric tons (sometimes written as tonnes and equal to
1,000 kg or 2,200 pounds [lb]) or short tons (2,000 lb); most of the rest
of the world uses metric tons. This report uses short tons when discussing
coal use in the United States.
o A ton of typical coal contains about 22 MJ of energy.
o A tonne of typical coal contains about 24 MJ of energy.
• Petroleum and gasoline supply and consumption quantities are
expressed in the United States in gallons or barrels (1 barrel = 42 gallons)
and internationally in liters (3.88 liters = 1 gallon). In the United States,
the energy content of liquid fuel is expressed in British thermal units (Btu),
million Btu (MMBtu or 106 Btu), and quadrillion Btu (quad = 1015 Btu).
The rest of world uses joules (J) to express the energy content of liquid
fuels (1 Btu = 1,055 J). A Btu is defined as the amount of energy (in the
form of heat) needed to raise the temperature of 1 lb of water by 1 degree
Fahrenheit.1 The energy content of different fuels can be converted to Btu
using the following approximate factors:
o 1 barrel crude oil = 5,800,000 Btu = 5.8 MMBtu
o 1 barrel gasoline = 5.2 MMBtu
o 1 barrel fuel ethanol = 3.5 MMBtu
• When different liquid fuels and blends are compared, this is often
done on the basis of what volume would give the same energy as a gallon of
gasoline. Therefore, about 1.5 gallons of ethanol would provide the energy
equivalent of 1 gallon of gasoline.
• Natural gas supply and consumption usage is expressed in units
of a thousand cubic feet (MCF or mcf). This is the equivalent volume of
gas at atmospheric pressure and temperature. Here the prefix M stands for
1A joule is the amount of energy needed to heat a kilogram of water by 1 degree centigrade.
1,055 joules = 1 Btu.

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40
COMMON UNITS AND CONVERSIONS
a thousand, and MM is used to denote a million cubic feet. One MCF of
natural gas contains about a million Btu of thermal energy.
Basis for Quantifying Impacts
• Activity-specific impacts result from particular energy use. For ex-
ample, impacts from the emissions from an electric power plant or impacts
from tailpipe emissions from a passenger car.
• Activity-aggregate impacts are used to describe the impacts from
energy use in a set of activities that include all impacts starting with the
processing of primary energy, its conversions and its transportation to its
end use point, its use to provide a set of energy services, and impacts as-
sociated with disposal of end use equipment. The aggregations are based on
life-cycle assessment (LCA) methods and use a variety of data and models
to estimate the impact. For example, electricity use to provide light in a
building would include all the “upstream inputs” to produce feed energy
for the power plant (mining, dams, etc.), the electricity production inputs
to generate and distribute power to the site of the light bulb, and impacts
associated with operation of the light bulb. Waste heat from the bulb and
its disposal would be “downstream impacts.” Larger downstream impacts
would be associated with the health and other consequences from emissions
at the power plant.
In this report, life-cycle impact assessment (LCIA) is a goal that can
only be achieved incompletely due to limitations in data availability and
complexity of the detailed systems, but where important impacts are pres-
ent their magnitudes are estimated to the extent possible.
Waste Streams and Hazardous Air Emissions
• Solid and liquid wastes are usually described using familiar units of
volume or weight per unit time or quantity of energy produced. (cubic feet
per minute [cfm]; tons per MWh; gallons per day; etc.). Where these waste
streams contain contaminants, the concentration of the contaminant of con-
cern is also important. (parts per million [ppm] by weight is the weight of
contaminant in a million units of carrier weight; or pounds of contaminant
per ton of carrier, or pounds of contaminant per gallon of liquid,)
• Air emissions are usually described by emissions per unit of en-
ergy produced or used—such as lb per MWh of electricity, lb per MCF of
natural gas, or grams per vehicle miles traveled (VMT) and sometimes in
terms of concentration of pollutants in emissions stream—such as parts
per million (by volume) or pounds per cubic foot. The choice of a VMT
basis is a compromise, since the more meaningful metric of passenger miles

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408 HIDDEN COSTS OF ENERGY
traveled would require information about the number of passengers per
vehicle—and would only change the final result if more passengers on
average travelled on vehicles powered by a particular fuel. Presentation
of results per gallon of fuel makes for difficult comparisons since different
fuels have different energy contents per gallon.
In this report, impacts are assessed nationally using detailed models
for the overall activities. Using a VMT basis for the transportation emis-
sions estimates includes not only the differences in the impacts for different
fuels, but also includes differences in the size and weights of vehicles that
constitute the national vehicle fleet.
Greenhouse Gases
• Carbon dioxide (CO2) emissions from energy production and use
are expressed in tons (short tons) or metric tons (tonnes) of CO2-equivalent
(CO2-eq). Although CO2 is the principal greenhouse gas associated with
energy use, other gases such as methane, nitrous oxide, black carbon, and
SF6, also make some contributions to warming potential. These other con-
tributions are converted to an equivalent amount of CO2 with a similar
effect and the total is therefore expressed as tonnes of CO2-equivalent. The
United States emits about 7 billion tonnes of CO2-equivalent per year, about
6 billion of which is CO2 arising primarily from energy production and use.
Average annual CO2 emissions in the United States are about 20 tonnes per
person. [Note: Sometimes greenhouse gas emissions are reported in terms
of tonnes of “carbon.” One tonne of carbon emissions equals 3.7 tonnes
of CO2 emissions, since the weight of CO2 also includes the weight of the
oxygen in the molecule.]

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